The present invention is directed to the protection of lithographic masks from particle contamination, particularly to a removable pellicle for lithographic mask protection, and more particularly to a removable pellicle for a lithographic mask utilizing magnetic coupling and thermophoretic protection.
Patterned lithographic masks are utilized in semiconductor chip fabrication, and such lithographic masks need to be protected from particle contamination since foreign matter on a mask will produce a printed defect in the electronic circuit being created on a silicon wafer. For current lithographic manufacture of semiconductor chips, masks are enclosed in a xe2x80x9cpelliclexe2x80x9d to protect them from particles. The mask consists of a rigid substrate with a patterned absorbing film on one surface. A pellicle is a thin membrane (typically organic-based but can be inorganic-based as well), stretched over a frame mounted to the mask substrate, which prevents particles from striking the patterned areas of the mask. The pellicle is offset from the mask in an xe2x80x9cout of focusxe2x80x9d image plane, producing a gap between the mask surface (requiring protection) and the pellicle. This offset ensures that particles intercepted by the pellicle do not produce image defects. For the photon wavelengths currently used in chip manufacture (365 nm, 248 nm), the pellicle is highly transparent and allows the lithographic radiation to be transmitted to the mask with high efficiency. Pellicles stay affixed to the mask mounting hardware throughout the life of the mask and allow the mask to be handled and inspected free from defect producing particle contamination.
The next generation of lithographic techniques, 157 nm optical projection lithography, extreme ultraviolet lithography (EUVL), ion projection lithography (IPL), and electron projection lithography (EPL), utilize ionizing radiation (photons, ions and electrons, respectively) to perform lithographic imaging. Thus, the masks used in these next generation lithographic (NGL) techniques are irradiated with ionizing radiation during the lithographic exposure. A traditional pellicle cannot be used with NGL lithography because the pellicle would absorb too much of the ionizing radiation. A membrane might also degrade in the ionizing beam, eventually failing and contaminating the mask. Furthermore, a traditional pellicle would not survive transfer from atmospheric pressure into the vacuum environment (xcx9c10xe2x88x926 Torr) needed for the NGL techniques. A sealed pellicle/mask assembly would have a trapped air space, which would burst through the thin membrane when the mask was placed in a vacuum environment.
Since the prior known pellicle is incompatible with NGL techniques only during the lithographic exposure, it is desirable to use a pellicle during the inspection and handling steps of mask manufacture at atmospheric pressure but to remove the pellicle before lithographic exposure. This raises a challenge to remove the pellicle without contaminating the mask with particles created by removal of the pellicle.
The present invention enables removal of a pellicle from an NGL mask without particle contamination of the mask. The removable pellicle of this invention enables its use in storage, handling and loading into NGL apparatus, but removal prior to lithographic exposure of the mask. The removable pellicle of the invention allows handling and inspection of the mask at atmospheric pressure, incorporates thermophoresis to prevent particle contamination, and is vacuum compatible so that it can be inserted into the vacuum environment of NGL exposure tools without bursting the pellicle membrane. After lithographic use, the removable pellicle may be placed back on the mask without particle contamination and transferred again from vacuum to atmospheric pressure without bursting of the pellicle or particle contamination of the mask.
It is an object of the present invention to provide a removable pellicle for a lithographic mask. A further object of the invention is to provide a lithographic mask pellicle which is removable prior to exposure to ionizing radiation.
A further object of the invention is to provide a removable pellicle for masks utilized in EUV, IP, EP, and 157 nm optical projection lithography.
A further object of the invention is to provide a removable pellicle containing a thin membrane that is usable during the inspection and handling of a lithographic mask at atmospheric pressure but can withstand vacuum applications and is removed before lithographic exposure of the mask.
Another object of the invention is to provide a removable pellicle for a lithographic mask which utilizes mechanical latching or magnetic attraction to removably retain the pellicle over the mask.
Another object of the invention is to provide a removable pellicle for a lithographic mask which utilizes thermophoresis to prevent particles from contaminating the mask.
Other objects and advantages of the present invention will become apparent from the following description and accompanying drawings. The present invention is directed to removable pellicles for lithographic masks. The invention utilizes mechanically latched or magnetically attracted members to retain the pellicle over the mask and which allows the pellicle to be removed from the mask without particle contamination of the mask. This invention enables the use of conventional thin membranes for use as pellicles for masks utilized in lithographic techniques that utilize ionizing radiation (photons, ions, and electrons) such as 157 nm, EUVL, EPL, and IPL. The removable pellicle of the invention is particularly applicable for extreme ultraviolet lithography (EUVL) and can be utilized in storage, handling, and loading into lithographic machines and which is removed prior to lithographic exposure of the mask and replaced following exposure of the mask. The removable pellicle is vacuum compatible and thus can be inserted into the vacuum environment of lithographic tools without bursting the pellicle membrane. Particle protection of the mask is also provided by thermophoresis in combination with a labyrinthine array to prevent particles from contaminating the mask during removal and replacement of the removable pellicle. The magnetic means for retaining the pellicle over the mask may be of a contacting or non-contacting arrangement. Thus, the removable pellicle of this invention utilizes a traditional pellicle and two deployments of thermophoretic protection to keep particles off the masks.